A) Field of the Invention
The present invention relates to an IC socket used for inspection of IC devices, such as IC packages, and specifically to an IC socket usable for inspection of IC packages having a plurality of lead terminals arranged at a fine pitch.
B) Description of Prior Art
An IC socket has a number of contact members, known as contact pins, arranged at the same pitch as that of a lead terminal array of an IC device to be tested. Each contact pin is formed to be brought into contact, at one end thereof, with a lead terminal of an IC device, while the other end of the contact pin is made to be electrically connected with a printed circuit board on which the IC socket is to be mounted. In order to assure steady contact of the IC device end of a contact pin with a lead terminal of the IC device loaded in the IC socket, there is formed a spring section between the IC-device end and the circuit-board end of the contact pin to allow the IC-device end to be movable. The contact pin usually is made to have a thickness of more than 0.1 mm, because a thinner contact pin would have difficulty in creating sufficient elastic force in the spring section. Usually, a thin division wall (hereinafter referred to as a rib) is provided near the IC-device end of each contact pin so that adjacent contact pins do not contact one another. In some cases, these ribs are integrally formed with the IC socket body out of synthetic resin. In recent years, however, a separate assembly of synthetic resin, in which a plurality of ribs are arranged, is joined to the IC socket body. Such an assembly is called a floating member, for it is reciprocated in a direction of insertion/removal of IC devices each time one IC device is replaced by another. When ribs having approximately 3 mm height are to be formed in one row, 0.2-0.3 mm is usually considered as a lower limit thickness of the ribs. There are, however, many problems to be solved in manufacturing ribs having thickness and inter-rib size less than this limit value and thus prior attempts to provide an inexpensive IC socket have been unsuccessful.
On the other hand, IC devices have been developed so as to be further integrated and further reduced in size, thus rapidly making their lead terminals arranged at more and more fine pitch. At the present stage, development of IC sockets cannot keep pace with the rapid compact-sizing of IC devices as to provide practical and inexpensive IC sockets for inspecting the IC devices. Therefore, in order to ease adaptation of an IC socket solutions have been proposed in which lead terminals, specifically those of TAB (Tape Automated Bonding) packages are extended to have otherwise unnecessary length and an arranged zigzag in two rows around four sides of the IC chip, as disclosed by Japanese Patent Preliminary Publication No. Hei 6-188339, thereby making interterminal pitch per row less fine. Such an arrangement, however, does not provide a satisfactory solution, thus TAB packages in which lead terminals are an arranged zigzag in three or four rows are under consideration.
A primary cause which prevents development of a practical and inexpensive IC socket for inspection is that it is difficult to form practical ribs to be used as mentioned above out of synthetic resin. Recent IC devices may have, at finest, the interterminal pitch of 0.2-0.25 mm, with lead terminals of 0.1-0.15 mm width, which size substantially coincides with the thickness of the contact pins of the IC socket above-mentioned. Accordingly, the airspace between adjacent lead terminals becomes 0.1-0.15 mm. Therefore, it is physically impossible to form the ribs having the above mentioned thickness so that they are accommodated in these airspaces.
To enable an IC socket to cope with the fine interterminal pitch of IC devices, two typical methods are conventionally known. According to one method, contact pins themselves are processed so that an insulating film is locally formed on one side or both sides thereof, while no member intervenes between the contact pins. According to another method, a thin metal plate of 0.6 mm thickness having a polyamide film glued on one or both surfaces thereof, is arranged between contact pins.
To apply the former method, in the first process, pieces shaped as contact pins are stamped by a press machine out of a hoop material on which an insulating film is locally formed in such a manner that the pieces are not completely cut away off the hoop material. Then a plated layer is formed without break over a predetermined portion of the pieces as contact pins by plating. In the last process, the pieces are completely cut away off the hoop material to form individual unit devices. As described above, this method involves a considerable number of manufacturing processes. Moreover, process control of this method for assuring uniform thickness over an entire contact pin is very difficult, because the insulating film is liable to bend off. On the other hand, since the latter method requires the thin minute metal plates to be fixed one by one manually, assembly costs can increase dramatically.